Environmental interests and an increasing demand for diesel fuel, especially in Europe, encourage fuel producers to employ more intensively available renewable sources. In the manufacture of diesel fuels based on biological raw materials, the main interest has concentrated on vegetable oils and animal fats comprising triglycerides of fatty acids. Long, straight and mostly saturated hydrocarbon chains of fatty acids correspond chemically to the hydrocarbons present in diesel fuels. However, neat vegetable oils display inferior properties, particularly extreme viscosity and poor stability and therefore their use in transportation fuels is limited.
Conventional approaches for converting vegetable oils or other fatty acid derivatives into liquid fuels comprise transesterification, catalytic hydrotreatment, hydrocracking, catalytic cracking without hydrogen and thermal cracking among others. Typically triglycerides, forming the main component in vegetable oils, are converted into the corresponding esters by the transesterification reaction with an alcohol in the presence of catalysts. The obtained product is fatty acid alkyl ester, most commonly fatty acid methyl ester (FAME). Poor low-temperature properties of FAME however limit its wider use in regions with colder climatic conditions.
Said properties are the result of the straight chain nature of the FAME molecule and thus double bonds are needed in order to create even bearable cold flow properties. Carbon-carbon double bonds and ester groups however decrease the stability of fatty acid esters, which is a major disadvantage of transesterification technology. Further, Schmidt, K., Gerpen J. V.: SAE paper 961086 teaches that the presence of oxygen in esters results in undesirable higher emissions of NOx, in comparison to conventional diesel fuels.
Undesired oxygen may be removed from fatty acids or their esters by deoxygenation reactions. The deoxygenation of bio oils and fats, which are oils and fats based on biological material, to produce hydrocarbons suitable as diesel fuel products, may be carried out by catalytic hydroprocessing, such as hydrocracking, but also more controlled hydrotreating conditions may be utilized.
During hydrotreating, particularly hydrodeoxygenation oxygen containing groups are reacted with hydrogen and removed through formation of water and therefore this reaction requires rather high amounts of hydrogen. Due to the highly exothermic nature of these reactions, the control of reaction heat is extremely important. Impure plant oil/fat or animal fat/oil, high reaction temperatures, insufficient control of reaction temperature or low hydrogen availability in the feed stream may cause unwanted side reactions, such as cracking, polymerisation, ketonisation, cyclisation and aromatisation, and coking of the catalyst. These side reactions also decrease the yield and the properties of diesel fraction obtained.
Unsaturated feeds and free fatty acids in bio oils and fats may also promote the formation of heavy molecular weight compounds, which may cause plugging of the preheating section and decrease catalyst activity and life.
The fatty acid composition, size and saturation degree of the fatty acid may vary considerably in feedstock of different origin. The melting point of bio oil or fat is mainly a consequence of saturation degree. Fats are more saturated than liquid oils and in this respect need less hydrogen for hydrogenation of double bonds. Double bonds in a fatty acid chain also promote different kinds of side reactions, such as oligomerisation/polymerization, cyclisation/aromatisation and cracking reactions, which deactivate catalyst, increase hydrogen consumption and reduce diesel yield.
Plant oils/fats and animal oils/fat may contain typically 0-30% of free fatty acids, which are formed during enzymatic hydrolysis of triglycerides especially when oil seeds are kept in humid atmosphere. Free fatty acids can be also formed during purification of bio oils and fats, especially during caustic wash i.e. alkali catalyzed hydrolysis. The amount of free fatty acids present in plant/vegetable oils is typically 1-5 wt % and in animal fat 10-25 wt-%. Free fatty acids are corrosive in their nature, they can attack against materials of unit or catalyst and can promote some side reactions. Free fatty acids react very efficiently with metal impurities producing metal carboxylates, which promote side reaction chemistry.
Fatty acids may also promote the formation of heavy compounds. The boiling range of these heavy compounds is different from the range of diesel fuel and may shorten the life of isomerisation catalyst. Due to the free fatty acids contained in bio oils and fats, the formation of heavy molecular weight compounds are significantly increased compared to triglyceridic bio feeds, which have only low amount of free fatty acids (<1%).
Biological raw materials often contain metal compounds, organic nitrogen, sulphur and phosphorus compounds, which are known catalyst inhibitors and poisons inevitably reducing the service life of the catalyst and necessitating more frequent catalyst regeneration or change. Metals in bio oils/fats inevitably build up on catalyst surface and change the activity and selectivity of the catalyst. Metals can promote some side reactions, but blocking of catalyst active sites typically decreases the activity and thus metal impurities such as Na, Ca, and Mg compounds should be removed as efficiently as possible.
Hydrolysis of triglycerides produces also diglycerides and monoglycerides, which are partially hydrolyzed products. Diglycerides and monoglycerides are surface-active compounds, which can form emulsions and make liquid/liquid separations of water and oil more difficult. Bio oils and fats can also contain other glyceride-like surface-active impurities like phospholipids (for example lecithin), which have phosphorus in their structures. Phospholipids are gum like materials, which can be harmful for catalysts. Natural oils and fats also contain other types of components, such as waxes, sterols, tocopherols and carotenoids, some metals and organic sulphur compounds as well as organic nitrogen compounds. These compounds can be harmful for catalysts or pose other problems in processing.
U.S. Pat. Nos. 4,992,605 and 5,705,722 describe processes for the production of diesel fuel additives by conversion of bio oils into saturated hydrocarbons under hydroprocessing conditions with CoMo and NiMo catalysts. The process operates at high temperatures of 350-450° C. and produces n-paraffins and other hydrocarbons. The product has a high cetane number but poor cold properties (melting point>20° C.), which limits the amount of product that can be blended in conventional diesel fuels in summer time and prevent its use during winter time. The formation of heavy compounds with a boiling point above 343° C. was observed, especially when a fatty acid fraction was used as a feed. A reaction temperature with a lower limit of 350° C. was concluded as a requirement for trouble-free operation.
A two-step process is disclosed in FI 100248, for producing middle distillates from vegetable oil by hydrogenating fatty acids or triglycerides of vegetable oil origin using commercial sulphur removal catalysts, such as NiMo and CoMo, to give n-paraffins, followed by isomerising said n-paraffins using metal containing molecular sieves or zeolites to obtain branched-chain paraffins. The hydrotreating was carried out at rather high reaction temperatures of 330-450° C., preferably 390° C. Hydrogenating fatty acids at those high temperatures leads to shortened catalyst life resulting from coking and formation of side products.
EP 1 396 531 describes a process containing at least two steps, the first one being a hydrodeoxygenation step and the second one being a hydroisomerisation step utilizing counter-current flow principle, and using biological raw material containing fatty acids and/or fatty acid esters as the feedstock. The process comprises an optional stripping step.
Deoxygenation of plant oils/fats and animal fats with hydrogen use a large amount of hydrogen and at the same time releases significant amount of heat. Heat is produced from deoxygenation reactions and from double bond hydrogenation. Different feedstocks produce significantly different amounts of reaction heat. The variation of reaction heat produced is mainly dependent on double bond hydrogenation. The average amount of double bonds per triglyceride molecule can vary from about 1.5 to more than 5 depending on the source of bio oil or fat.
FR 2,607,803 describes a process for hydrocracking of vegetable oils or their fatty acid derivatives under pressure to give hydrocarbons and to some extent acid. The catalyst contains a metal dispersed on a support. A high temperature of 370° C. did not result complete oxygen removal or high selectivity of n-paraffins. The product mixture formed, contained also some intermediate fatty acid compounds.
Formation of water during hydrotreatment results from the deoxygenation of triglyceride oxygen by the means of hydrogen (hydrodeoxygenation). Deoxygenation under hydrodeoxygenation conditions is to some extent accompanied by a decarboxylation reaction pathway and a decarbonylation reaction pathway. Deoxygenation of fatty acid derivatives by decarboxylation and/or decarbonylation reactions forms carbon oxides (CO2 and CO) and aliphatic hydrocarbon chains with one carbon atom less than in the original fatty acid molecule. Decarb-reactions mean here decarboxylation and/or decarbonylation reactions.
The feasibility of decarboxylation varies greatly with the type of carboxylic acid or derivative thereof used as the starting material. Alpha-hydroxy, alpha-carbonyl and dicarboxylic acids are activated forms and thus they are more easily deoxygenated by decarb-reactions. Saturated aliphatic acids are not activated this way and generally are difficult to deoxygenate through decarb-reactions.
Decarboxylation of carboxylic acids to hydrocarbons by contacting carboxylic acids with heterogeneous catalysts was suggested by Maier, W. F. et al: Chemische Berichte (1982), 115(2), 808-12. Maier et al tested Ni/Al2O3 and Pd/SiO2 catalysts for decarboxylation of several carboxylic acids. During the reaction the vapors of the reactant were passed through a catalytic bed together with hydrogen. Hexane represented the main product of the decarboxylation of the tested compound heptanoic acid. When nitrogen was used instead of hydrogen no decarboxylation was observed.
U.S. Pat. No. 4,554,397 discloses a process for the manufacture of linear olefins from saturated fatty acids or esters, suggesting a catalytic system consisting of nickel and at least one metal selected from the group consisting of lead, tin and germanium. With other catalysts, such as Pd/C, low catalytic activity and cracking to saturated hydrocarbons, or formation of ketones when Raney-Ni was used, were observed.